Heat resistant steel

ABSTRACT

A steel composition particularly useful for components of solid oxide fuel cells, for example a connector plate for collecting electrical current from a solid oxide fuel cell, consisting of, in weight percent, 18-28.5 Cr, 0.001-0.20 C, &lt;0.1 Si, 0.005-0.10 Mn, &lt;1.0 Ni, &lt;0.25 N, &lt;0.05 S, &lt;0.08 P, &lt;0.06 Al, &lt;0.25 Additional Defined Metals, and 0.005-0.50 REM, wit the residue iron, excluding incidental impurities, where Additional Defined Metals is the sum of titanium, niobium, vanadium, molybdenum and copper, and REM is one or more of the rare earth metal elements in the group of the lanthanide elements 57 to 71, scandium and yttrium.

This application is a 371 of PCT/AU98/00956 filed Nov. 17, 1998.

The present invention relates to a heat resistant steel, and isparticularly concerned with such a steel for use in components of solidoxide fuel cells.

The operating conditions in a solid oxide fuel cell are very severe onmost metals, causing them to degrade via loss of mechanical strength,oxidation or other form of corrosion, distortion, erosion or creep.Various heat resistant metals have been developed to cope with many ofthese forms of degradation. Most such metals are alloys based on iron ornickel with substantial additions of chromium, silicon and/or aluminium,plus, in some alloys, more expensive elements such as cobalt, molybdenumand tungsten. Chromium based metals are also available.

These alloys are either expensive to make and fabricate, unsuitable forlong term use in certain components in fuel cells, or both. A relativelycheap iron based steel which is sufficiently suitable for critical fuelcell components has not hitherto been developed.

The significant feature of all heat resistant steels is the oxide layer,particularly its type and nature, which is formed when the steel isexposed to mildly and strongly oxidising conditions at elevatedtemperatures. Heat resisting steels form tight, adherent, dense oxidelayers which prevent further oxidation of the underlying metal. Theseoxide layers are composed of chromium, aluminium or silicon oxides orsome combination of these. These oxide layers are very effective inproviding a built-in resistance to degradation due to high temperatureoxidation.

However, while this feature is used to advantage in many applications,the presence of this oxide layer inhibits the use of these steels in keycomponents of solid oxide fuel cells. The oxides, especially those ofsilicon and aluminium, are electrically insulating at all temperatures,and this is a major problem for those components in a fuel cell whichmust act as electrical current collectors. Of all the heat resistingsteels available, those based on the iron-chromium binary systems arethe best in this regard, but they too have severe limitations.

The currently available steels contain additional elements which havebeen found to affect the nature of the oxide layer when it forms. Theseelements are present in small quantities, either as deliberate additionsto assist with the control of oxygen during steelmaking, or as residualimpurities inherited from the raw materials used in making the steel,i.e. tramp elements. Many of these minor elements have a profound effecton the type and thickness of the oxide layer which forms on the surfaceof the steel when it is subjected to oxidation at elevated temperatures.For example, manganese is deliberately added to most steels to assistwith the deoxidation of the iron during melting and to eliminate ironsulphides from the steel. This is beneficial for most applications ofheat resistant steels, but not when the steel is used as an interconnector connector plate in a solid oxide fuel cell.

In a paper in “Nature” Feb. 13, 1965, vol.205, p.609, Caplan and Cohenreported that in tests of high temperature oxidation rates on Fe-26Cralloys, those with manganese levels around 0.003 to 0.004 weight percentoxidised slower than those with levels of 0.75 to 1.00 percentmanganese. The applicant has now found that the presence of manganese inquantities above 0.10 percent by weight modifies the form of the oxidelayer as it begins to grow, giving rise to a rather loose and wavylayer. This results in a particularly poor electrical conductivitythrough the layer, at the stage of formation and at a later stage whenthe composition may have shifted to one of the other more stable oxidessuch as chromium oxide. The applicant has also found that the beneficialproperties of low oxidation rates can be achieved at manganese levelsmuch higher than the 0.003 to 0.004 weight percent levels of Caplan andCohen, providing the manganese level is kept below 0.10 weight percentand providing the inclusion of certain other elements is also limited.This higher permissible manganese level allows the production ofcommercial tonnages of steel at a reasonable price.

Another example is the effect which the element silicon has on theformation of oxide layers at the surface of the steel. Silicon andaluminium are used as cheap and effective additives to control theoxidation of iron during the steel smelting process. Small amounts ofsilicon, e.g. 0.5 weight percent, in an iron chromium heat resistingsteel lead to the formation of a subsurface layer of silica which, iffully formed, has a very high electrical resistivity. For mostapplications this feature is not deleterious, but for a connector platein a solid oxide fuel cell it completely negates a prime purpose of thecomponent.

For these heat resisting steels to be useful for electrical conductingcomponents in fuel cells, it is important that the aforementioneddisadvantages be alleviated.

According to the present invention there is provided a steel compositionconsisting of, in weight percent,

Chromium   18-28.5 Carbon 0.001-0.20 Silicon <0.1  Manganese 0.005-0.10Nickel <1.0  Nitrogen <0.25 Sulphur <0.05 Phosphorus <0.08 Aluminium<0.06 Additional Defined Metals <0.25 REM 0.005-0.50

with the residue iron, excluding incidental impurities, and whereAdditional Defined Metals is the sum of titanium, niobium, vanadium,molybdenum and copper.

Further according to the present invention, there is provided, in asolid oxide fuel cell stack, a component adapted to be exposed to atemperature in excess of 750° C. and an oxidising atmosphere, saidcomponent being formed of a steel composition in accordance with theinvention.

Still further according to the invention, there is provided a connectorplate for collecting electrical current from a fuel cell, said platebeing formed of a steel composition in accordance with the invention.

The deleterious effects of the significant minor elements in known heatresistant steels have been alleviated according to the invention bycontrolling their levels to subcritical values. Any incidentalimpurities not specifically identified in accordance with the inventionmay be present, but any such presence should be at no more than tracelevels.

In addition, REM elements, which are not usually present in commonlyavailable steels, are included, singly or in combination, in the steelcomposition according to the invention. REM is hereby defined as meaningany one or more of the rare earth metal elements in the group of thelanthanide elements 57 to 71, scandium and yttrium, and are preferablypresent to a total level in the range of 0.01 to 0.25 wt %. The presenceof REM in these small, precisely controlled amounts helps stabilise theoxide layers at a much reduced thickness and improved adhesion and henceassists in reducing the electrical resistivity of the oxide scale on thesurface of the steel component.

Steels made according to the invention form a stable, adherent and verythin layer of chromium oxide which protects the underlying metal fromfurther oxygen induced degradation and provides a level of electricalconductivity which is substantially superior to that possible withsimilar steels manufactured to their accepted specifications.

Preferred and more preferred composition ranges in the present inventionare as follows:

Preferred More Preferred Chromium   20-27.5 23-25 Carbon 0.01-0.080.03-0.06 Silicon <0.1  <0.09 Manganese 0.005-0.05  0.005-0.05  Nickel<0.1  <0.02 Nitrogen <0.20 <0.10 Sulphur <0.03 <0.01 Phosphorus <0.04<0.04 Aluminium <0.05 <0.05 Additional Defined Metals up to 0.10 <0.10REM 0.01-0.25 0.01-0.10

residue, iron excluding incidental impurities.

Steels made to a composition in accordance with the invention yield fuelcell component performances which may be superior to those obtainablewith other heat resistant metals currently available, in one or more ofthe following properties:

a. Cost Per Unit Mass

The unit cost of steels in accordance with the invention is less thanthat of other materials such as nickel alloys, austenitic stainlesssteels, chromium alloys and ceramics which have been used forinterconnect plates in solid oxide fuel cells.

b. Toughness

The composition of the invention results in ferritic steels which are astough, i.e. resistant to cracking, as most other ferritic steels andmuch tougher than all of the ceramics and the chromium based alloys suchas “Ducrolloy” produced by Metallwerk Plansee GmbH. Typical toughnessvalues, expressed as “elongation to fracture in a tensile test”, are12-25% for ferritic stainless steels and 0-0.5% for Ducrolloy. A hightoughness level has a great advantage during fabrication, forming (intoshapes), assembly, and refurbishment, since a tougher metal is able totolerate small elastic and plastic strains far better than can brittlematerials.

c. Hot Ductility

The composition of the invention results in a steel which has theability to plastically deform under relatively low loads at typicalsolid oxide fuel cell operating temperatures. This plastic “compliance”ensures a good general contact between the contiguous surfaces of fuelcell components under light loads, a contact which greatly improves theelectrical performance of the fuel cell and reduces any high load pointsthat may exist. These high load points in fuel cell components such asconnector plates can lead to cracking of the relatively brittleelectrolyte plates which in turn may allow the direct mixing of fuel andair at the fracture. The resultant fire can lead directly to the failureof the fuel cell assembly. Other materials such as Ducrolloy and allceramics do not possess this property of plastic compliance to anyuseful extent and the contacting surfaces of plates made from suchmaterials have to be carefully prepared to enable good contact to bemade. Because the steel made according to the present inventionpossesses good ductility and a lower mechanical strength, it is not asprone to the phenomenon of “springback” as is Ducrolloy and many otherheat resistant steels. This property renders the present steel lessprone to unwanted distortion during the operation of the fuel cellassembly, especially during temperature fluctuations, start up and cooldown.

d. Oxidation Resistance

The steel produced according to the invention has an excellent, inherentresistance to surface degradation at temperatures within the range 500°C.-950° C. in the atmospheres usually present in a solid oxide fuelcell, namely moist air, moist hydrogen, moist hydrocarbons and oxides ofcarbon. This oxidation resistance is approximately equal to that ofDucrolloy and to that of all other commercial stainless steels exceptthose which contain at least 4.5% by weight of aluminium. (It is to benoted that once the aluminium level of a steel exceeds approximately4.5% by weight, the oxide layer which forms on exposed surfaces of thesteel is composed of alumina which has a very low electricalconductivity.) The superior oxidation resistance of steels made to thecomposition of the invention can be further enhanced by suitablechemical treatment of the surface by methods such as calorising orcoating with a protective layer of another material.

e. Thermal Expansion Compatibility with Zirconia

At typical operating temperatures, i.e. 700° C.-1000° C., the thermalexpansion coefficient (CET) of the steel according to the invention iswithin 10% of that of the partially stabilised zirconia which is thebasic material of solid oxide fuel cell electrolytes. This means thatthermally induced strains do not give rise to stresses which aresufficient to cause cracks in the cells. In this regard the steelcomposition of the invention is as good as any other metal used forinterconnect plates and superior to many.

f. Machinability

The steel made to the composition of the invention is easily machined byconventional metal cutting techniques and in this regard is superior tothe austenitic, martensitic and dual phase stainless steels, as well asto the nickel based alloys and chromium based alloys currently used forinterconnect plates.

g. Weldability

The steel of the invention is readily weldable without specialpreparation, electrodes or equipment, and without pre- or post-heating.This property makes it more convenient to fabricate with conventionalindustrial techniques and easier to repair and modify than other alloyssuch as Ducrolloy.

Steels having a composition in accordance with the invention are notrestricted to any particular processing techniques, including forcooling from melt temperatures, and may be processed and/or treatedsimilarly to other ferritic steels such as grade 446 steel.

Batches of steels made to the above broadest specification have provento be effective in fuel cells tested by the applicant, and to besuperior in performance to alloys made to currently acceptedspecifications. For example, a solid oxide fuel cell stack composed of50 cells, each with a nominal size of 150×150 mm, was constructed withinterconnect plates machined from plates of Ducrolloy supplied byPlansee. When operated, it suffered from distortion of the interconnectplates, leading to cracking of the electrolyte plates and poorelectrical contacts, and yielded a maximum power output of approximately0.75 kW at 930° C. The Ducrolloy interconnect plates were so brittleafter service in the fuel cell stack that none was able to bedisassembled intact.

Experimental batches of twenty two different steels according to thepresent invention were manufactured and tested over a period of morethan a year. Analyses of the steels are shown in Table 1. While thelaboratory equipment used to measure yttrium levels was incapable ofdetecting levels below 0.01 wt %, in view of the method used to producethe steels it is believed that the five steels identified in Table 1with Y levels <0.01 wt % in fact contained about 0.005 wt % yttrium.This series of tests culminated in a test on a stack of solid oxide fuelcells as described below using fuel cell interconnect plates made fromfifteen of the above twenty two batches all falling within the followingweight percent ranges:

Cr 26.25-28   C 0.0025-0.090  Si 0.01-0.09 Mn 0.01 Ni 0.01 N <0.001 S0.001-0.002 P  0.002 Al 0.007-0.056 Ti + Nb + V + Mo + Cu <0.04  REM0.005-0.15, 

residue iron, excluding incidental impurities any of which were at tracelevels or below.

The fifteen steel compositions used in this stack test are indicated inTable 1 by having a Fabrication Number assigned at the top of the table.

Forty seven interconnect plates, variously made from these fifteen steelcompositions, were formed and assembled into a stack which was identicalto the stack of the above comparative example (which used the Ducrolloyplates) except that fifty plates were used in the comparative example.Under the same operating conditions, this stack of cells produced amaximum power output of 1.55 kW. All interconnect plates were removedintact from this stack and all were suitable for refurbishment andreuse.

The compositions of the interconnect plates of the present inventiontested in the stack test varied for some elements across a significantrange and the spread of compositions illustrates the scope of theinvention. A major cause of the variation in compositions was the natureof the small ingot fabrication facility which meant that a number ofbatches had to be made and it proved difficult to make them of theidentical composition with the available facilities. Except for thechromium and carbon levels, the majority of the composition ranges fellwithin the above more preferred composition range. With regard to thechromium levels, it is recognized that similar performance advantageswould be achieved at the lower, more preferred amounts of 23-25 wt %with additional cost savings.

Steel batches 3824, 3825, 3826, 3832, 3837 and 3841 proved difficult tohot roll for fabrication of the interconnect plates and were not usedfor the stack test. The difficulty arose because chromium and carbon areprone to combine during solidification or cooling phases of manufactureand to form a chromium carbide phase which may render the steelunworkable if it is present in sufficient volume. If the wt % chromiumis at the higher end of the given range and the carbon is likewise nearthe upper limit, care is required during subsequent processing to avoidthe formation of carbide initiated cracking of the steel. If however thesteel is able to be processed, its performance is satisfactory.

In the more preferred range of composition, the upper levels of chromiumand carbon are lower, and hence the danger of chromium carbide formationis greatly reduced: the steel is more readily processed and itsperformance is also satisfactory.

If any of the Additional Defined Metals group of elements is present, itis preferred that this be mostly or completely titanium because of costconsiderations. However it is believed that the effect of titanium isdue to the formation of titanium carbo-nitrides and, as the otherelements in the Additional Defined Metals group are known to havesimilar properties, they may be substituted in whole or in part for thetitanium if wished.

With respect to the REM elements, the various steels used in the stacktest Example all use yttrium as the only REM element. However the otherelements in the REM group as defined are known to have a similar effectin stabilising the oxide layers at a reduced thickness and could besubstituted for yttrium if wished. The five steels analysed at less than0.01 wt % yttrium gave improved performances compared to correspondingcompositions without any yttrium, but detectively lower performancesthan equivalent steels in Table 1 with higher yttrium levels.

In addition to interconnect plates, the steel of the invention may beused for other components of fuel cells, particularly solid oxide fuelcells, such as manifolds, base plates, current collector straps andducting. The steel may also be used in other fields requiring one ormore of the aforementioned desirable properties, such as heat exchangerplates, hot gas ducting, vanes, connectors, piping and tubing.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described. It is to be understood that the inventionincludes all such variations and modifications which fall within itsspirit and scope.

TABLE 1 Analysis of Experimental Steels Batch Mixture No. 10 11 12 38163824 3825 3826 3827 3828 3829 3830 Batch Fabrication No. 02/1 01/1 —02/5 — — — 02/1 04/3 01/1 04/1 Cr 27.40 26.75 18.15 27.15 27.40 26.1026.75 26.55 26.80 26.50 26.60 C 0.0031 0.0025 0.036 0.011 0.095 0.1000.095 0.090 0.090 0.065 0.070 Si 0.02 0.03 0.01 0.02 0.02 0.02 0.02 0.020.01 0.02 0.01 Mn 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01Ni 0.010 0.010 0.38 0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.010 N<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001<0.001 S 0.001 0.001 0.018 0.001 0.001 0.001 0.001 0.001 0.001 0.0010.002 P 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.0020.002 Al <0.05 <0.05 <0.05 0.042 0.036 0.041 0.038 0.036 0.020 0.0250.007 Ti <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01<0.01 Nb <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01<0.01 V <0.01 <0.01 <0.01 <0.01 0.009 0.009 0.007 0.007 0.008 0.0060.006 Mo 0.005 0.005 0.005 0.003 <0.002 <0.002 <0.002 <0.002 <0.002<0.002 <0.002 Cu 0.002 0.002 0.002 0.003 <0.002 <0.002 <0.002 <0.002<0.002 <0.002 <0.002 Y <0.01 0.02 <0.01 0.09 0.07 0.10 0.10 0.10 0.090.09 0.01 Ti + Nb + V + Mo + <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04<0.04 <0.03 <0.03 <0.03 Cu Cr + 30xC 27.50 26.83 19.23 27.48 30.25 29.1029.6 29.25 29.5 29.05 28.70 Batch Mixture No. 3832 3833 3835 3836 38373838 3840 3841 3842 3843 3844 Batch Fabrication No. — 04/5 02/3 04/7 —01/2 01/2 — 02/3 01/3 01/2 Cr 26.10 28.00 26.90 27.45 27.55 26.30 27.0026.45 26.65 26.25 27.60 C 0.130 0.075 0.075 0.080 0.090 0.080 0.0750.130 0.060 0.070 0.049 Si 0.05 0.02 0.03 0.03 0.03 0.06 0.06 0.06 0.050.06 0.09 Mn 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Ni0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.010 N<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001<0.001 S 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.0010.001 P 0.002 0.002 0.002 0.002 0.004 0.002 0.002 0.002 0.002 0.0020.002 Al 0.021 0.031 0.027 0.032 0.036 0.056 0.047 0.055 0.045 0.0520.04 Ti <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01<0.01 Nb <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01<0.01 V 0.007 0.007 0.007 0.008 0.006 0.011 0.011 0.017 0.011 0.009 0.01Mo <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002<0.002 Cu 0.002 <0.002 <0.002 0.003 <0.002 <0.002 <0.002 <0.002 0.002<0.002 <0.002 Y 0.05 0.15 0.15 0.10 0.15 0.09 <0.01 <0.01 0.01 <0.010.01 Ti + Nb + V + Mo + <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.040 <0.05<0.04 <0.04 <0.04 Cu Cr + 30xC 30.00 30.25 29.15 29.85 30.25 28.70 29.2530.35 28.45 28.35 29.07

What is claimed is:
 1. A steel composition consisting of, in weightpercent, Chromium   20-27.5 Carbon 0.001-0.20  Silicon <0.1  Manganese0.005-0.10  Nickel <1.0  Nitrogen <0.25 Sulphur <0.05 Phosphorus <0.08Aluminium <0.06 Additional Defined Metals <0.25 (total amount) REM0.005-0.50  (total amount)

with the residue iron, excluding incidental impurities, where AdditionalDefined Metals is any titanium, niobium, vanadium, molybdenum and copperpresent in the steel composition, and REM is any rare earth metalelement or elements in the group of the lanthanide elements 57 to 71,scandium and yttrium, said steel having a stable, adherent and very thinlayer of chromium oxide which protects the underlying metal from furtheroxygen induced degradation and provides a superior level of electricalconductivity for use in components of solid oxide fuel cells.
 2. A steelcomposition according to claim 1 consisting of, in weight percent,Chromium   20-27.5 Carbon 0.01-0.08 Silicon <0.1  Manganese 0.005-0.05 Nickel <0.1  Nitrogen <0.20 Sulphur <0.03 Phosphorus <0.04 Aluminium<0.05 Additional Defined Metals up to 0.10 REM 0.01-0.25

with the residue iron, excluding incidental impurities.
 3. A steelcomposition according to claim 2 consisting of, in weight percent,Chromium 23-25 Carbon 0.03-0.06 Silicon <0.09 Manganese 0.005-0.05 Nickel <0.02 Nitrogen <0.10 Sulphur <0.01 Phosphorus <0.04 Aluminium<0.05 Additional Defined Metals <0.10 REM 0.01-0.10

with the residue iron, excluding incidental impurities.
 4. A steelcomposition according to claims 1, 2, or 3 wherein the total level ofany Additional Defined Metals is <0.04 weight percent.
 5. In a solidoxide fuel cell stack, a component adapted to be exposed to atemperature in excess of 750° C. and an oxidising atmosphere, saidcomponent being formed of a steel composition according to claims 1, 2,or
 3. 6. A connector plate for collecting electrical current from asolid oxide fuel cell, said plate being formed of a steel compositionaccording to claims 1, 2, or
 3. 7. A connector plate according to claim6 wherein at least one surface thereof is subjected to a calorisingtreatment.